The M113 details will work only for the front part of the hull, the exhaust, the headlights guards and the rear door handle, that are the parts the 577 shares with them. Personally I wont buy them if I'm not planning to build an M113, 'cause the most part of the PE set will remain unused. If you plan to build a 113 then you can buy the set and clone the parts you need for the M577 "cloning" the cutting parts you need cutting some copper foil or aluminium can (the one from soda, cola, and so on)
Ciao